FIG. 1 illustrates a variable geometry nozzle 3 which modulates the high velocity gases passing through it, as in the propulsion system of a jet aircraft 6. Such nozzles are commonly activated by hydraulic pistons, commonly called rams (not shown in FIG. 1), which are powered by pressurized hydraulic fluid. A system of servovalves commonly controls delivery of hydraulic fluid to the rams. A schematic of one system now in use is shown in FIGS. 2 and 3.
The fundamental goal of the system shown in those figures is to move an actuating arm 9, which is fastened to rams 11 and 13, to known, selectable positions along an axis 16. Motion of the arm 9 is achieved by application of hydraulic fluid through conduits 18A-D into cylinders 20-22 to move rams 11 and 13. The position of the arm 9 along the axis 16 is detected by a position sensor 24. The components on the left of dashed line 26 in FIGS. 2 and 3 form generally a mirror image of those to the right of this dashed line. The functioning of the former will be described, with the understanding that the functioning of the latter is analogous, and both together control the position and motion of rams 11 and 13.
A master control valve 27 includes a modulator piston 30 contained within a cylinder 33. The piston moves left and right under the influence of hydraulic fluid applied by conduits 36 and 38. As the modulator piston 30 moves left and right, spools 39A-C either cover or uncover conduits 41A-C. These conduits are respectively connected to a return manifold designated R.sub.AB, a pressure manifold designated P.sub.AB, and the aforementioned return manifold R.sub.AB. An example of master control valve motion is shown in FIG. 3. As the modulator piston 30 is moved leftward, in the direction of arrow 44, the spool 39B is also moved, uncovering conduit 41B, thus allowing hydraulic fluid indicated by arrows 46 to flow from the pressure manifold P.sub.AB along conduit 41B and into cylinder 20, thus pushing ram 11 to the left. This action moves the actuator arm 9 leftward also, since the arm 9 is attached to the ram 11.
The hydraulic fluid applied to the modulator piston 30 is controlled as follows. A pressure reservoir P.sub.SAB in FIG. 2 supplies pressurized fluid to each of servovalves 50A and 50B through conduits 51 and 52. One type of servovalve which can be used is that described in U.S. Pat. No. 4,276,809, invented by the Applicant. Each valve 50A and B contains respectively a movable jet pipe 55A and B which selectively directs a stream of pressurized fluid (not shown) to respective receiver conduits 58A-D. As described in the patent just identified, the pressures in the conduits 58A-D depend upon the relative positions of the jet pipes 55 with respect to receiver conduits 58A-D.
The jet pipes 55 are rotated by means of a torque motor 60. Coils 62A-D produce a magnetic field which reacts with the magnetic field produced by permanent magnets 64A and B to thereby generate a moment about point 68, thus bending the jet pipes 55A and B as shown in FIG. 3. The amount of fluid pressure reaching the cylinder 33 depends upon the amount of bending of the jet pipe 55B. This fluid pressure 70 drives the modulator piston 30 to the left, thus moving the spools 39A-C as shown to admit fluid pressure 46 into cylinder 20 to move the ram 11 as described above.
External circuitry (not shown) coordinates the electric current applied to coils 62A-D of 50A-B in FIG. 2 in response to signals produced by sensors 24 and 24A to appropriately rotate the jet pipes 55A-B so that the proper pressures are applied conduits 58A-D in order to move the modulator piston 30 in the desired direction, at the desired speed, and in the desired amount.
Only one of each position sensors 24 and 24A is shown in FIG. 2. This is for ease of illustration only. It is to be understood that the driving circuitry (not shown) for each servovalve 95 in FIG. 4 has its own position sensor, so that the two servovalves 95A and 95B each have one position transducer for the modulator piston and one transducer for the ram piston, for a total of four transducers.
A monitor 75 monitors the pressure at point 76 in a manifold 78 which is connected to the receiver conduits 58B-C of each of servovalves 50A and B in FIG. 2. The monitor 75 also monitors the pressures in conduits 36 ad 38 at points 80 and 81. The monitor 75 compares these three pressures and if they deviate from a predetermined schedule of pressures, a fault in the servovalve is assumed to have occurred, and the monitor 75 disconnects power from a solenoid 82 by means of a signal which the monitor 75 supplies to Bus A, thereby disconnecting the pressure source P.sub.SAB from the jet pipes 55A-B. Thus, no pressure is applied to conduits 36 and 38 and the movement of the master control valve 27 is then undertaken solely by the valve system to the right of the dashed line 26. This second valve system to the right of dashed line 26 is identical to that on the left, as stated above. This second valve system controls the modulator piston 85 which drives the master control valve 27 which, in turn, controls ram 13.
The system shown in FIGS. 2 and 3, while having many desirable characteristics, can have disadvantages in some aircraft applications. For example, in aircraft in the category designated short takeoff and landing (STOL), the thrust of the engine is vectored up-, down-, left-, right-, forward and rearward by mechanisms which are actuated by systems resembling those of FIGS. 2 and 3. This STOL application can require as many as eleven such actuation systems. Given that the components located above dashed line 88 in FIG. 3 can weigh 35 lbs and can cost many tens of thousands of dollars, the requirement of eleven such systems clearly imposes a cost and weight penalty.
In addition, if the monitor 75 detects an improper pressure differential, as for example caused by a malfunction of servovalve 50A, the system removes both valves 50A and B from operation, irrespective of the possibility that one of them may still be functional.